The combination of a conventional distillation process with an appropriate adsorption system to dry ethanol reduces overall energy requirements in producing fuel-grade ethanol1-3. Adsorption processes break the ethanol-water azeotrope and remove final amounts of water. Starch-based adsorbents have been proven as an energy-efficient desiccant to remove water from alcohols with advantages of low cost, selectivity for water molecules, and mild regeneration conditions4-8. The mechanism of water adsorption by starch is based on the formation of hydrogen bonds between water molecules and hydroxyl groups on the starch. The first industrial use of corn grits as starch based adsorbent was a result of joint effort between Purdue University and Archer Daniels Midland (ADM) in 1984. Unlike commercial inorganic adsorbents such as molecular sieves or silica gels, corn grits are biologically based, biodegradable, non-toxic and derived from renewable biomass. In addition, the spent adsorbent itself can also serve as feedstock for producing ethanol.
Dehydration of 92 to 93% (by weight) ethanol to yield fuel-grade ethanol (99.5% ethanol by weight) on an industrial scale is done by using a fixed bed adsorption system. Water is selectively removed from a hydrous ethanol vapor by corn grits during the adsorption cycle and the corn grit bed is regenerated by hot CO2 gas (˜96° C.) that is counter-currently passed through the bed during regeneration. The system consists of two or more packed adsorption beds, of which one is under adsorption mode while at least one other is in regeneration mode. Heat of water adsorption given off during the feed cycle is stored in the bed and used to dry the bed during the regeneration cycle. Other types of regenerant gas, such as nitrogen, could be used for the bed regeneration, but CO2, which is a co-product of ethanol fermentation, is chosen as it is readily available from an ethanol plant9.
There have been extensive studies on the equilibrium and kinetic aspects of water removal by various types of starch adsorbents at a laboratory scale. The starch adsorbents studied include native corn grits10,11, modified corn grits12, corn meal5,6,13-15, wheat flour16, synthesized starch-based adsorbent17, and cassava (manioc) starch pellets18. The mechanism of water adsorption, equilibrium kinetics and chemistry of maize-derived starch particles, such as corn grits and corn meal, as desiccants are discussed in many literatures. However, little is known about the capacity of cassava starch for use as a drying agent. The application of cassava starch generally as an adsorbent for water in alcohol mixtures was first reported by Carmo et al. (2004)18. They have shown the effectiveness of cassava starch on the liquid phase adsorption of water from various alcohol-water mixtures. The results demonstrated a high affinity for cassava starch to absorb water from various alcohols. However, adsorptive ethanol drying in industry is usually carried out under hot, vapor phase conditions that can alter the properties of starch absorbing particles.
Cassava starch is the fourth largest source for starch production after corn, wheat, and potato19, and is abundantly produced in various tropical regions. Its world production is approximately 192 million tons per year22.
It would therefore be beneficial to discover whether any forms of cassava starch might be particularly useful alternatives to corn grits for industrial scale dehydration of ethanol. In addition, it would be further beneficial to define the structural morphology of the fraction of cassava starch particles that conveys the most effective drying properties so as to be able to form starch particles from other sources having that morphology. The present disclosure provides such a discovery as well as a description of the forms of starch and methods of using the same that are particularly suitable for dehydrating ethanol made by fermentation.